1. Field of the Invention
The invention relates to a steam generator having a convective part arranged downstream of the furnace chamber and essentially consisting of final superheater, pre-superheater, evaporator and economizer connected in series on the flue-gas side, a further contact heat exchanger having a plurality of tube banks being arranged upstream of the final superheater.
2. Discussion of Background
Steam generators of this type are known, for example from the article "Empfehlungen zum Bau optimaler Eckrohr-Mullkessel", published in the journal "Brennstoff, Warme, Kraft", volume 38 (March 1986). In order to keep the contamination on account of high tube-wall temperatures within limits and restrict the corrosion problem at high superheated-steam temperatures, the flue-gas temperature upstream of the superheater is not to substantially exceed approximately 650.degree. C. Therefore an evaporator is provided as contact heat exchanger as first contact surface downstream of the radiation-pass outlet. Here, the evaporator arranged upstream of the superheater should have an open transverse spacing and contain so many tube rows that the flue-gas temperature does not exceed the said approximately 650.degree. C. upstream of the superheater.
The critical part in the design of the convective part of a steam boiler, for example for refuse incineration, consists in the correct dimensioning of these heating surfaces, that is of the contact heat exchanger, acted upon first by the flue gas. This so-called "protective nest" is as a rule designed for the contaminated operation at high flue-gas temperatures in such a way-that the aforesaid 650.degree. C., which the operator of the installation is guaranteed, upstream of the superheater is not exceeded. The superheater in turn is as a rule designed for clean operation at guaranteed minimum load, i.e. a predetermined partial load. This is an operating point at which relatively cold flue-gas temperatures prevail at the radiation-pass outlet as a result of the temperature gradient in the radiation space. However, since the relatively amply proportioned heating surfaces of the contact heat exchanger are still arranged upstream of the superheater, the already relatively cool gases are additionally cooled down there. Therefore the superheater has to be of large and complicated construction in order to actually reach the guaranteed superheater temperature. In contaminated operation, on the other hand, i.e. when the flue gases at the radiation-pass outlet are substantially hotter, the superheater of correspondingly large size receives too much heat. A remedy is provided here by virtue of the fact that, at the superheater, which as a rule is a two-part superheater, a large quantity of water is injected between the pre-superheater and the final superheater. This can lead to a second costly injection possibly having to be provided. Or else the so-called running time, i.e. the interval between two boiler inspections, must be reduced for the purpose of cleaning the exchange tubes.